The present invention relates to a process for producing .epsilon.-caprolactam from cyclohexanon oxime using a solid acid as a catalyst under conditions of gas phase reaction.
.epsilon.-caprolactam is an important raw material for nylon and the like. One of the processes for producing the caprolactam is liquid phase rearrangement of cyclohexanone oxime using fuming sulfuric acid or concentrated sulfuric acid as a catalyst.
However, the above process has the problems that a large amount of fuming sulfuric acid is required and besides ammonium sulfate is by produced in a large amount.
As an approach to solve these problems, various processes have been proposed according to which cyclohexanone oxime is rearranged in gas phase using a solid acid as a catalyst. For example, there has been proposed use of boric acid catalyst (Japanese Published Unexamined Patent Application No. 37686/78 and Japanese Published Examined Application No. 12125/71), silica alumina catalysts (British Patent No. 831,927), solid phosphoric acid catalyst (British Patent No. 881,926), mixed metal oxide catalysts (Journal of the Chemical Society of Japan, No. 1, 77, 1977), and zeolite catalysts (Journal of Catalysis, 6, 247, 1966 and U.S. Pat. No. 4,359,421.
However, the above processes have problems in reaction selectivity of the objective .epsilon.-caprolactam, life of catalysts, productivity per catalyst and quality of the product .epsilon.-caprolactam. For example, U.S. Pat. No. 4,359,421 discloses an example of using a crystalline zeolite such as ZSM-5 having an atomic ratio of Si/Al of 40-60 and mentions that conversion of cyclohexanone oxime is quantitative, but is silent about selectivity of .epsilon.-caprolactam. Besides, it shows a short catalyst life of 15-20 hours. The present inventors have also studied use of ZSM-5 zeolite having the Si/Al atomic ratio as mentioned in the above patent and have found that not only the life of catalyst, but the selectivity of .epsilon.-caprolactam are not enough.
U.S. Pat. Nos. 4,709,024 and 4,717,769 show use of a crystalline aluminosilicate having an Si/Al atomic ratio of at least 500 and an acid amount on external surface of less than a specific value and a crystalline metallosilicate having an Si/metal atomic ratio of at least 500. Selectivity of these catalysts has been considerably improved as compared with conventional silica catalysts. Furthermore, U.S. Pat. No. 4,717,770 discloses that when crystalline zeolite catalysts are surface treated with an organometallic compound, selectivity can be improved.